1. Field of the Invention
This invention relates to the disproportionation of alkylaromatic feedstreams and, more particularly, to the disproportionation of toluene containing feedstocks employing a metal-promoted, steam-modified omega zeolite catalyst.
2. Description of the Related Art
The disproportionation of toluene involves a well-known, catalyzed transalkylation reaction in which toluene is converted to benzene and xylene in accordance with the following reaction: ##STR1## Reaction (1) is mildly exothermic. Crystalline aluminosilicates, or zeolites, are well-known in the art and have found extensive application as hydrocarbon catalysts. While many zeolites occur naturally, more than 40 species of synthetic crystalline zeolites are known to have been prepared within the past decade. These synthetic compositions are distinguishable from each other and from the naturally occurring zeolites on the basis of factors such as composition, crystalline structure, adsorption properties and, perhaps most importantly, characteristic x-ray powder diffraction pattern. Zeolites are of an ordered crystalline structure comprising "cages" or cavities occupied by large ions and water molecules, both of which have considerable freedom of movement, permitting ion exchange and reversible dehydration. Access to these cavities or "channels" is gained by way of orifices within the crystalline lattice. These openings limit the size and shape of molecules that can be adsorbed. A separation of mixtures of molecules based upon molecular dimensions, whereby certain molecules are adsorbed by the zeolite while the entry of others is prevented, is therefore possible. It is this characteristic property of many crystalline zeolites that has led to their designation as "molecular sieves." For a general discussion of zeolite catalysts, reference is made to Kirk-Othmer, Encyclopedia of Chemical Technology, 3rd Edition, 1981 under the heading "Molecular Sieves", Vol. 15, pages 638-643.
In addition to molecular size and shape, however, other factors may also influence the selective adsorption of certain foreign molecules by molecular sieves. Among these factors are: the polarizability and polarity of the adsorbate molecules; the degree of unsaturation of organic adsorbates; the size and polarizing power of the interstitial cation; the presence of adsorbate molecules within the crystalline lattice (interstitial spaces); and the degree of hydration of the zeolite.
In addition to the unique adsorption properties of zeolite molecular sieves, certain of these materials, particularly when chemically modified, are effective catalysts in hydrocarbon conversion processes such as reforming, cracking, isomerization, dehydrogenation and the like. Because the mechanisms involved in these catalytic applications are complex, however, the precise chemical properties of the zeolites which contribute to a particular catalytic activity are not fully understood.
As indicated, many catalytic applications have been discovered for certain zeolites. Among the more widely used and studied zeolite forms are: mordenite, beta and ZSM-5. Lesser known and studied omega zeolite, which is the catalyst employed in the present invention, was first identified by Flanigen, et. al. in U.S. Pat. No. 4,241,036. According to Flanigen, et. al. omega zeolite is not only a distinct species of zeolite molecular sieve but also is a member of a new structural class of zeolites exhibiting a unique and previously unknown framework arrangment of SiO.sub.2 and Al.sub.2 O.sub.3 tetrahedra. While Flanigen, et. al. discloses only the composition of omega zeolite and various procedures by which the composition can be prepared, the inventors did indicate that various cation and decationized forms of zeolite omega could be effective in the hydrocarbon conversion processes commonly referred to as cracking, hydrocracking, isomerization, polymerization, hydrogenation, reforming and paraffin alkylation. This indication notwithstanding, however, and as compared to the aforereferenced and more widely used zeolites, the catalytic properties of omega zeolite are considerably less well known or studied. Low thermal stability has been the reason cited most frequently for the dearth of investigative activity respecting omega zeolite.
According to extant scientific literature, omega zeolite may be destroyed or may undergo a considerable decrease in crystallinity when calcined at temperatures exceeding 600.degree. C. While a number of explanations have been advanced, the reason for the thermal brittleness of omega zeolite remains not well understood. Despite this uncertainty and the variety of postulations, however, recent studies directed toward improving the thermal stability of omega zeolite have been successfully conducted. For example, in Volume 4 of the work entitled "The Synthesis and Thermal Behaviour of Zeolite" (1984) pp. 263-269 by Araya, Abraham, et. al., it is reported that the small quantity roasting of NaTMA omega form in an apparatus of differential thermal analysis leads to a solid which remains crystallized at temperatures up to 800.degree. C. This solid, however, is not dealuminated and retains all initial alkali cations. Earlier work involving the roasting of an NH.sub.4 TMA omega compound was reported by Weeks, et. al., in an article appearing in the Journal of the Chemical Society entitled "Thermochemical Properties of Ammonium Exchanged Type Omega Zeolite," Farad Trans 1, 72(1976), 57. Despite some observed thermal stabilization, however, the solid compound failed to demonstrate desirable catalytic activity when tested in hydrocracking and isomerization applications.
In addition to attempts to thermally stabilize the compound, the dealumination of omega zeolite has also been the focus of some investigative activity. U.S. Pat. No. 3,937,791 to Garwood, et. al. discloses the dealumination of various zeolites, including omega zeolite, by Cr (III) salts. This method leads to replacement of the aluminum atoms by chromium atoms. Notwithstanding that the structure is dealuminated, its chromium content is also fatally increased. U.S. Pat. No. 4,297,335 to Lok, et. al. recommends a dealumination technique by treatment with fluorine gas at high temperature. This treatment is applicable to various zeolites but, when applied to omega, it results in degradation of the crystalline structure. European Patent No. 100,544 to Gortsema, et. al. discloses the dealumination of many zeolites, including the omega form, by roasting in the presence of SiCl.sub.4 at temperatures lower than 200.degree. C., despite that higher temperatures are known to be required for dealumination in accordance with such technique (Beyer, et. al. Catalysis by Zeolites, (1980) p. 203). The dealumination of omega zeolite by SiCl.sub.4 is in fact possible but only at temperatures, for example, above 500.degree. C. as disclosed by J. Klinowski, et. al. JCS, Chem. Commun. 1983, p. 525 and O. Terasaki, et. al. Proc. R. Soc. London (A), 395 (1808), 153-64). Treatment of the omega zeolite by this technique, however, results in a virtually negligible increase in the SiO.sub.2 to Al.sub.2 O.sub.3 ratio. Moreover, inasmuch as dealumination by treatment with SiCl.sub.4 is applicable to omega zeolite it is essential to note that this technique results in irremediable replacement of the aluminum atoms of the structure with silicon atoms (H. Beyer, et. al. Catalysis by Zeolites, B. Imelik, et. al. editors (1980), p. 203, Elsevier Amsterdam).
Despite the investigative activity described above, the most important advance concerning efforts to both thermally stabilize and dealuminate the omega zeolite form has apparently been made by Raatz, et. al. as disclosed in U.S. Pat. No. 4,724,067. Therein, it is claimed that a practical and useful hydrogen form of omega zeolite can be prepared by a process involving alternating ion exchanges and acid etchings with thermal treatments. Raatz, et. al. further disclose that their process yields a thermally stable, dealuminated omega zeolite in hydrogen form which functions as an active and selective catalyst in cracking and hydrocracking reactions.
While it has been discussed that much investigative work has occurred involving the use of zeolites as catalysts, it is clear that the use of the omega form of the composition remains not fully understood. Moreover, and perhaps due in some measure to the compound's reputation for thermal instability, its known application in the field of alkylaromatic catalysis has been limited to cracking and hydrocracking reactions.
In view of the foregoing, it is clear that a need in the art exists for a means of employing omega zeolite in high level conversion of aromatic hydrocarbons.